Elimination of corona in electrical apparatus



Oct. 7, 1952 R. w. WIESEMAN 2,613,238

ELIMINATION OF CORONA IN ELECTRICAL APPARATUS FiledJuly 27, 1951 Figl.2.

Fig. Z.

Inventor:

FEobert W. Wieseman.

His Attokney.

Patented Oct. 7, 1952 ELIMINATION OF CORONA IN ELECTRICAL APPARAT s vRobert W. Wieseman, Schenectady, N. Y.,,assignor to General ElectricCompany, a corporation of New York Application July 27, 1951, Serial No.238,887

3 Claims. (01. l71252) My invention relates to constructions andprocesses which have significance in connection with the problem ofeliminating corona between an insulated conductor and an adjacentmetallic j member.

Heretofore, it has been known to coat the sur face of conductorinsulation with a semi-conducting material such as a paint, lacquer orvarnish containing dispersed particlesof a conductor such as metal orgraphite. Thus it has been known in the past to paint the surface ofinsulated conductors lying in core slots of large electrical machineswith such a-semi-conducting material in order to establish aroundtheconductor a sheath of reasonably uniform potential. While this hasproved satisfactory in many installations, for some applications andparticularly in the newer machines of larger size and of higher voltagethan those formerly known, such a procedure has not entirely eliminatedthe along the winding slot, nor-entirely eliminated electricaldischarges at the end portions where the coil projects beyond themagnetic core. Such corona and discharges are objectionable from theformation of corona adjacent localized points standpoint of creatingradio interference, and;- even if they do not reach such proportions asto be damaging to insulation they cause the creation of ozone, a verysmall trace'of which is readily perceivable by smell and this issometimes objectionable from the standpoint of customer acceptance.

It is an object of the present invention to provide simple andinexpensive means for overcoming the above-mentioned difiiculties. "A'further object of the presentinvention is to provide an improved methodof constructing electrical apparatus so as to prevent formation spacesbetween coil surfaces and slot surfaces because the slots vary in widtha small amount, and the slot portion widths of the coils vary a smallamount, and a small clearance space is required to place the coils inthe slots without damaging the coil insulation.

0f corona, reduce radio noise and reduce dielec- It is, therefore, astill further object of the present invention to provide in adynamoelectric machine having a stator comprising interinsulatedlaminationsprovided with winding slots with aninsulated winding in saidslots, means for eliminating corona and ozone formationregardless ofmanufacturing clearances and consequent small air spaces between coiland slot walls. Another object of the present invention is toprovidemeans for reducing end discharges which occur when a coil slotsheath is not at ground potential. a In accordance with one aspect of myinvention, I completely eliminate or minimize the corona eifect in largehigh voltage alternating current dynamoelectric machines by not onlycoating the insulated conductors with a semiconducting material but alsoscuiiing'the slot sides and thereafter applying semi-conducting materialto such sides along the lamination edges to insure that allsemi-conducting sheaths are sufficiently grounded by contact with eachother and with the core. s

The features of my invention which I believe to be novel are set forthwith particularity in ever, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawing in V longitudinal sectionshowing a portion of a dynamoelectric machine stator core provided withwinding slots with a winding therein and incorporating the presentinvention.

Referring to the drawing, the stator core is 'made; up of a plurality ofpunchings ill each provided with a plurality of winding slots H and eachprovided with interlamination insulating varnish I2. As is well known inthe art, the varnish is applied as by dipping the individual punchingsafter the slots have been formed and the burrs removed, for if the slotswere punched out after the varnishing operation, the lamina- -tionswould be short circuited.

ductor is first sprayed or otherwise coated with a semi-conducting paintor other semi-conducting material 16 before the insulator conductor isplaced in the slot. This semi-conducting coating extends throughout theslot region and for a creepage distance therebeyond at each end of thecore as indicated at la in Fig. 2. In accordance with the presentinventionjeachsl'ot side is also first prepared before placing theinsulated and semi-conductor material coated conductor therein, byscufiing, as with a file, the lamination edges along the slot walls andthereafter, preferably immediately thereafter, applying a semiconductingpaint or varnish containing conductive graphite particles orother'semi-conducting material, to the slot walls soscufied to' providea semi-conducting coating H on the slot walls.

When the insulated conductor is thereafter placed in the slot as shownin the drawing, coating i1 is immediately adjacent the semi-conduct-'conductingrubbers, and it followsthat. the invention is not limited to aparticularmeans of applying, as with a brush or spray, sincethe'semiconducting material may be applied in any other manner, as bywinding of otherwise applying a semi-conducting tape or cloth mat to theconductors adjacent the slot portion and to the slot walls themselves.With the process and construction of the invention, the semi-conductingsheaths around the coil conductors are sufiiciently grounded by asatisfactory contactbetween the two coatings and with the core. Oneexplanation of the difficulties encountered with prior art arrangementsmay bethat the insulating varnish applied .to the core laminationsalways extended over the slot edges .adjacentthe slot' Walls and hadsufiicient ohmic resistance to prevent sufficient electrical contactbetween the slot surface andthe semi-conducting sheath on thearmaturecoiljso that the sheath is a fioatingjsheath with a varying potentialabove ground and, therefore, incapable of completely eliminating corona.With'th'e prior art arrangement, mere sending of theslotiwalls was notenough, possibly because the scuffed portions tendedto oxidize or rustthereby to eventually preventa good contact with the conductor sheath orto provide a sheath ground resistance which varied from slot to slot-.with the result that corona might form' in some slots and not inothers. it is, of course, desirable that the ohmic resistance or" asemi-conducting sheath applied to bare laminations be made high Inconnection with the development of the present invention, tests haverather conclusively indicated that itis not always suflicient to merelyiii "semi-conductor, localized corona, may be completely eliminated atall common operating voltages not only in the slot regions but, becausethe creepage portions of the conductor sheath are .well grounded, at.the end portions of the coils as well. There is thus provided aconstruction and method of the character described capable of 1 meetingthe objects hereinabove set forth,

coat the slot walls with semi-conducting paint" and leavesemi-conducting material off of the coil insulation. This is so becauseif there is. any

small air-space between the-insulation and the While I have illustratedand described a particular embodiment of my invention, modificationsthereof will occur to those skilled in the art. I desire it'to beunderstood, therefore, that my invention is not to be limited to theparticular arrangement disclosed and 'I intend in the appended claims tocover all modifications which do not depart from the true spirit andscope of my invention.

-What I claim as new and desire to secure by Letters Patent of theUnited States'is:

1. In electrical apparatus having a magnetic core member provided withwinding slots, the combination of a coating of a semi-conductingmaterial on the walls of said slots, high voltage insulatedconductors'located in said slots, 'and a semi-conducting coatingcovering the insulation of said conductors adjacent said coating ofsemiconducting material on the walls of said slots.

.2. In a dynamoelectric machine having a stator core portion comprisingvarnishinsulated laminationsprovided with winding slots, an insulatedwindinginsaid slots, said winding having a semiconducting coatingapplied to .the outer surface of the insulation thereof in the region of.said slots, and said slots having asemi-conducting coating applied tovarnish tree edges of said laminations along the'walls .of each slot,said two coatings being in substantial contact with one another toprovide a reasonably uniform potential sheathextending around saidwinding insulation in the slot region thereof with saidsheath-adequatelygrounded to said slot walls to prevent corona formation between saidwinding and said slot walls. I

3. In a high voltage dynainoclectric machine having a slotted corestructure, an insulated winding extending through the slots in said corestructure and projecting therefrom, a semi-conducting coating coveringthe insulation in each of the slot regions .of said winding andextending therebyond to provide a creepage distance-at each end of saidcore, and a semi-conducting coating covering the walls of said slots andincontact with said first mentioned coating to prevent the formation ofcorona-between said conductors and. said slotted core structure. v

ROBERT w. WIESEMAN.-

'No references cited.

